Typically when encoding information a modulation scheme is super imposed on a radio frequency carrier. The carrier is a sine wave. In amplitude modulation the information to be transmitted is used to strengthen or weaken the carrier to indicate the value of the information. Typically, in AM radio the information to be transmitted is not synchronized to the carrier frequency but is only used to modulate the carrier. For example in commercial AM radio analog information is used to directly modulate the transmit carrier, where the information to be transmitted is directly proportional to the power level of the transmitted signal. Due to the generation of beat frequencies, using the information signal to modulate the carrier results in three signals: the carrier, the carrier plus the modulating signal and the carrier minus the modulating signal. AM typically produces a center carrier with little information and two thirds of the power and two side bands on each side of the carrier that contain the encoded information but only one third of the power. For example, in traditional AM radio broadcasting, a 10 kHz signal is carried on a carrier between 600 kHz to 1700 kHz and occupies 20 kHz of bandwidth. The occupied bandwidth is twice the bandwidth of the information signal, or two times the band width of the audio signal. The data or information is not synchronized to the carrier phase angle or cycles. The advantage of the following disclosure is that it uses a very narrow bandwidth to carry very high rates of data. The bandwidth is not correlated to the data rate. The data rate equals the transmit frequency.
In the simplest form of AM communications a sine wave carrier signal is turned on and off to represent information. This is the system used to transmit Morse code. A carrier is keyed on and off to represent data in the form of long bits and short bits. The frequency of on off keying is usually much less than the frequency of the carrier, and the keying is not synchronized to the cycles of the carrier. The present invention switches the carrier on and off with the changes synchronous to the frequency of the carrier.
Other versions of AM just use one or both of the side bands while suppressing the main carrier. AM single side band is more bandwidth efficient since it only uses one of the sidebands, but is very prone to noise.
Cable TV systems along with others use QAM encoding. This is a combination of AM and FM modulation schemes. It uses two carriers that are 90 degrees out of phase with each other. Each of these two channels is phase and amplitude adjusted independently to represent information. The data is synchronized to these two lower frequency channels and 8, 16, 64, and 256 bps are typical encoding rates. After encoding the two channels, they are combined and then mixed with a high frequency sine wave to form the final transmit signal. The data is not synchronized to the phase angle of the transmit carrier, but is synchronized to the two internal 90 degrees out of phase intermediate frequency carriers of the QAM modulator which are significantly lower in frequency than the frequency of the transmit carrier. The bandwidth of the transmission is the symbol rate divided by the encoding rate.
In Frequency Modulation the information is encoded onto a lower frequency carrier called an intermediate frequency which is then mixed with a higher frequency signal to result in a transmit frequency signal. The transmit carrier is usually at a much higher frequency than the rate of the information to be carried and the carrier is not synchronized with the transmitted information. The band width is two times the information signal bandwidth.
Phase shift keying uses phase shifts in a sine wave signal to transmit binary states. Phase shifts can occur twice per cycle (BPSK) up to many times per cycle. Usually an intermediate frequency carrier is encoded with phase shifts and then combined with a high frequency carrier to result in a transmit frequency carrier.
None of the above systems synchronize the digital data to the frequency of the transmit carrier.
Van Nguyen in U.S. Pat. Nos. 6,462,679 B1 and 6,621,426 B1 suggested changing the amplitude of the carrier wave synchronously with the transmit carrier phase by selecting from one of several sine wave generators of different amplitudes, each level indicating a binary state. In U.S. Pat. No. 6,621,426 B1 he suggested switching between sine wave generators every half cycle and in U.S. Pat. No. 6,462,679 B1 he suggested switching between sine wave generators every ¼ cycle.
A problem occurs in the Van Nguyen patent when amplifying multiple carriers in a common amplifier as the carriers interact with each other. The full power signal of one sine wave will draw power from all other carriers reducing the amplitude of the other carriers, resulting in false zeros. Intermodulation occurs between the various carriers as each of the carriers change binary state. This results in false readings in said carriers, particularly as the bits per cycle increases or the number of carriers increases. The current invention avoids this by reducing the amplitude to near zero for one of the binary states with full power for the other sate. It can be looked at as on off signaling timed to the carrier phase. The presence of a carrier indicates a one and a near zero voltage carrier indicates a zero. Intermodulation will not effect the detection of the bit state of the carrier. Secondly, the Van Nguyen process of selecting from multiple carriers will result in noise at the switching between sine waves. Thirdly, the system proposed herein is much simpler to implement since it just requires the switching of a transistor on and off synchonized to the carrier. Fourthly, the current invention suggests only changing states once per carrier cycle rather than two to four times as suggested by Van Nguyen.
Mark Jorden in U.S. Pat. No. 6,574,284 B1 suggested changing the amplitude of a carrier on a microcontroller bus signal to represent two binary states by selecting between two sine wave signals. He suggested changing the amplitude twice per cycle. The resulting waveform looks like a carrier with increased and decreased amplitudes synchronized to each half cycle. He did not propose switching the carrier on and off. The problem of inter-carrier modulation is not addressed since the disclosure is for transmission along wires of a single data signal.
In order to avoid the consequence of other carriers in the same amplifier affecting the amplitude of the first carrier and to simplify the method of encoding, this disclosure is presented.
The advantages of transmitting one bit per one cycle of transmitted information are 1) a high data rate, and 2) narrow bandwidth, that is, any sideband signals are so far away from the main carrier that they are easily filtered off and all of the information is carried in the center frequency. The lower side band is actually at zero hertz. The bandwidth required is only that necessary to detect a sine wave carrier.
In the following descriptions and disclosure, the “carrier” refers to a sine wave at the transmit frequency unless otherwise indicated.